Cost competitive PSA (Pressure Swing Adsorption) system designs rely on high utilization of well matched system components including feed and vacuum blowers, vessel bed and adsorbents, valves, and other equipment. Safe, consistent, efficient use and control of this equipment is critical. To maintain control of this highly transient process, plants are equipped with Programmable Logic Controllers (PLCs) and computers running control and monitoring software. Recent development of radial flow adsorbers with short bed lengths, advanced adsorbents with high adsorption rates, and other system enhancements have acted to shorten cycle times, further adding increased process sensitivity to the transient nature of PSA operations.
Further, development of low pressure ratio VPSA cycles has facilitated a switch to single stage rotary lobe vacuum pumps. To be effective these pumps must be carefully sized to match other system components, such as bed volume and feed air blowers. These vacuum blowers are operated at pressure differentials that are near their maximums from a mechanical standpoint, and aerodynamically at a point where efficiency would fall sharply with increased vacuum levels.
In theory, once proper operating pressures are established for a system they do not need to be adjusted. A problem is that variations such as ambient temperature and pressure, control valve positioning, plant tuning parameters, operating temperature, cooling loops, blower and compressor mechanical wear and valve leaks will effect these pressure levels, and at times cause off-peak operation by forcing system components to operate away from their design points. In extreme cases, even equipment shutdown is possible. Therefore, for safe and efficient operation, the overall pressure levels of the system must be carefully monitored during plant operation, and when required, adjusted to nearly match desired values.
In cases where more than one adsorption vessel is utilized in a system, another problem may arise. This problem stems from difficulty in matching individual operation of the vessels in a manner which yields optimum system performance as measured by minimum unit power, minimum product purity variation, maximum production, etc. As is the case with overall system pressure level control, bed to bed balancing is required because disturbances enter the system and at times act to change system stability. Such disturbances can be introduced by variations in ambient temperature, ambient pressure, process equipment, process valve positioning, process valve response time, plant computer control functions, and others. Effects of these disturbances can be minimized by monitoring key process parameters, and then by making adjustments via the process control system to restore the system to near design operation. This bed to bed balance is required, in addition to the overall pressure level control mentioned above.
U.S. Pat. No. 4,747,853 to Haslett et al. describes a method of over pressurization control, for cases of valve failure, that utilizes a pressure sensing device, a flow restricting orifice and a normally open valve. If the pressure sensing unit detects unacceptable pressures, a signal is sent to the normally open valve, causing it to close. The downstream system is thus protected from the higher pressure in a manner similar to commercially available relief valves or bursting disks.
Bed balancing/tuning is described in U.S. Pat. No. 5,407,465 to Schaub, et.al. This patent recognizes a need to maintain bed to bed balance and concludes that a balanced system will operate with each bed having a similar axial temperature profile. If a disturbance enters the system, the axial temperature profile changes for each bed, providing an indicator of the unbalance. A plant control computer is utilized to monitor bed temperatures and to make adjustments to equalization and purge flows entering and exiting each vessel in a manner designed to restore similar temperature profiles to each bed.
U.S. Pat. No. 5,529,607 to Ziming Tan indicates that maximum O.sub.2 concentration measured in purge gas effluent from individual beds being operated out of phase in a cyclic PSA process, can be monitored. Then the absolute difference of the concentrations can be determined and adjustments made to the purge process step time of each bed in a manner which reduces the absolute value of the concentration difference.
U.S. Pat. No. 5,486,226 to Ross et al. uses an oxygen analyzer to measure impurity in a carbon PSA designed to make N.sub.2. If the O.sub.2 impurity rises above acceptable limits, flow of product quality gas is initiated from a surge tank into the adsorption vessels in a manner to restore product purity. This provides a means of rapid restart after an outage or other upset.
U.S. Pat. No. 5,258,056 to Shirley et al. describes a method for controlling output production from a plant by measuring a change in product demand and then adjusting the feed airflow to compensate for the change in product demand. Feed airflow is adjusted in such a manner as to maintain a constant product purity. The system controls feed air for both turndown capacity control and purity control.
U.S. Pat. No. 4,725,293 to Gunderson describes a system for control of impurity levels in a product stream by monitoring purity levels of the product stream and then by adjusting feed air flow in a manner to maintain desire product purity.
U.S. Pat. No. 4,693,730 to Miller et al. proposes a method for controlling the purity of a gas component in the product stream of a PSA. The concentration of impurity is monitored in a co-current depressurization, or equalization gas to determine if there is an upset condition present. If a problem is detected, then the process is adjusted. The main actions that can be undertaken to correct the purity problem are:
a) Change the adsorption step time to control the position of the leading edge of the mass transfer front. PA1 b) Change the end point of the final co-current depressurization step so that break through of the mass transfer front does not occur. PA1 c) Increase the amount of purge gas to each vessel.
In essence the Miller et al. system monitors purity at a time in the cycle when it is changing most quickly and when upsets are most easily detected. It is thus possible to detect events before they have a chance to fully propagate and show in the product purity.